Power storage device

ABSTRACT

An object is to increase the amount of ions capable of entering and leaving a positive electrode active material in an ion battery so that the capacity of the battery is increased. When a solid solution including alkali metal oxide having electrical conductivity less than or equal to 10 −10  S/cm and including alkali metal with a valence of 2 or more, and alkali metal oxide having electrical conductivity greater than or equal to 1×10 −6  S/cm and less than or equal to 3×10 −6  S/cm is used as a positive electrode active material in an ion battery, the amount of ions capable of entering and leaving the positive electrode active material is increased, so that the capacity of the battery is increased.

TECHNICAL FIELD

The present invention relates to a power storage device and anembodiment of the disclosed invention relates to the structure of apositive electrode of the power storage device.

BACKGROUND ART

In recent years, with an increase of consciousness about environmentalengineering, development of power generation devices using a powergeneration method which poses fewer burdens on the environment (e.g.,solar power generation device) than power generation devices usingconventional power generation methods has been actively conducted. Alongwith development of power generation devices, power storage devices havealso been developed.

As a power storage device, a secondary battery such as a lithium ionsecondary battery (alternatively called a lithium ion storage battery orsimply a lithium ion battery) can be given as example (see PatentDocument 1). Lithium ion secondary batteries have high energy densityand are widely popular because they are suited for miniaturization.

In a lithium ion secondary battery, lithium metal oxide is used for apositive electrode and a carbon material such as graphite is used for anegative electrode. As a positive electrode active material of a lithiumion secondary battery, for example, a positive electrode active materialincluding composite oxide containing at least alkali metal andtransition metal can be given.

In a lithium ion battery, at the time of charging, lithium in a positiveelectrode material is ionized into a lithium ion and moved into a carbonmaterial of a negative electrode material through an electrolytesolution. Generally, when the percentage of a material which ions canenter and leave is increased in an active material with the volume ofthe active material unchanged, the amount of ions capable of enteringand leaving the active material is increased, which can lead to anincrease in capacity of a battery.

[Reference]

[Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    H09-035714

DISCLOSURE OF INVENTION

An object of an embodiment of the present invention is to increase theamount of ions capable of entering and leaving a positive electrodeactive material so that the capacity of a battery is increased.

According to an embodiment of the present invention, a solid solutionincluding a material having electrical conductivity less than or equalto 10⁻¹⁰ S/cm and a material having electrical conductivity greater thanor equal to 1×10⁻⁶ S/cm and less than or equal to 3×10⁻⁶ S/cm is usedfor a positive electrode.

According to an embodiment of the present invention, a solid solutionincluding lithium metal oxide having electrical conductivity less thanor equal to 10⁻¹⁰ S/cm and lithium metal oxide having electricalconductivity greater than or equal to 1×10⁻⁶ S/cm and less than or equalto 3×10⁻⁶ S/cm is used for a positive electrode.

According to an embodiment of the present invention, a solid solutionincluding lithium metal oxide having electrical conductivity less thanor equal to 10⁻¹⁰ S/cm and including lithium with a valence of 2 ormore, and lithium metal oxide having electrical conductivity greaterthan or equal to 1×10⁻⁶ S/cm and less than or equal to 3×10⁻⁶ S/cm isused for a positive electrode.

According to an embodiment of the present invention, the solid solutionhas electrical conductivity greater than or equal to 1×10⁻⁷ S/cm andless than or equal to 10×10⁻⁷ S/cm.

When a solid solution including a material with high electricalconductivity and a material with low electrical conductivity is used asa positive electrode active material, the amount of ions capable ofentering and leaving the positive electrode active material can beincreased as compared to that in the case where either of the materialwith high electrical conductivity and the material with low electricalconductivity is singly used. Accordingly, the capacity of a battery canbe increased.

In addition, according to an embodiment of the present invention, alkalimetal oxide, which could not be used as a positive electrode materialbecause alkali metal is contained therein but is less likely to beionized, can be used as a positive electrode material. Thus, an ionbattery can be manufactured using alkali metal oxide, which is excellentin terms of safety and cost but could not be used.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a graph of the electrical conductivity of positive electrodematerials, according to an embodiment of the present invention;

FIG. 2 is a graph of voltage-capacity in a battery using a positiveelectrode of an embodiment of the present invention;

FIG. 3 illustrates an example of a battery which is an embodiment of thepresent invention; and

FIG. 4 illustrates an example of a cross section of a battery which isan embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the invention will be described withreference to the drawings. Note that the present invention can becarried out in many different modes, and it is to be easily understoodby those skilled in the art that the mode and details of the presentinvention can be variously changed without departing from its spirit andscope. Therefore, the present invention is not construed as beinglimited to description of the embodiment mode and embodiments. Notethat, in the drawings hereinafter shown, the same portions or portionshaving similar functions are denoted by the same reference numerals, andrepeated description thereof will be omitted.

[Embodiment 1]

In this embodiment, a positive electrode active material and a positiveelectrode including the positive electrode active material, which areembodiments of the present invention, will be described.

The positive electrode active material described in this embodiment isformed using a solid solution including first alkali metal oxide andsecond alkali metal oxide.

A high-resistance material, i.e., a material with low electricalconductivity is used as the first alkali metal oxide. As the secondalkali metal oxide, a material whose resistance is lower than the firstalkali metal oxide, i.e., a material with high electrical conductivityis used.

For example, as the first alkali metal oxide, a material havingelectrical conductivity less than or equal to 10⁻¹⁰ S/cm, such asLi₂MnO₃, LiMnPO₄, Li₂MnSiO₄, or Li₂FeSiO₄ is used. Further, it ispreferable to use a material including alkali metal with a valence of 2or more because alkali metal is more likely to be ionized as the amountof alkali metal included in a positive electrode material is increased.

As the second alkali metal oxide, for example, a material havingelectrical conductivity greater than or equal to 1×10⁻⁶ S/cm and lessthan or equal to 3×10⁻⁶ S/cm, such as LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ (alsorepresented as LiCoMnNiO₄), LiFePO₄, LiCoO₂, LiMn₂O₄, or LiNiO₂, ispreferably used. Note that in the first alkali metal oxide and thesecond alkali metal oxide, Li may be replaced with different alkalimetal such as sodium, and Mn or Fe may be replaced with differenttransition metal.

A solid solution is formed using the first alkali metal oxide and thesecond alkali metal oxide. The solid solution including the first alkalimetal oxide and the second alkali metal oxide has higher electricalconductivity than the first alkali metal oxide. Note that depending onmaterials, the electrical conductivity of the solid solution is greaterthan or equal to 1×10⁻⁷ S/cm and less than or equal to 10×10⁻⁷ S/cm.This is because the electrical conductivity of the solid solution isaffected by properties of the second alkali metal oxide.

When the solid solution is used for a lithium ion battery, the capacityof the battery is largely increased as compared to that in the casewhere either the first alkali metal oxide or the second alkali metaloxide is singly used for a positive electrode.

From the above phenomenon, it can be found that there is a closerelationship between electrical conductivity and capacity, and that whenthe solid solution is formed using a combination of the first lithiummetal oxide with low electrical conductivity and the second lithiummetal oxide with high electrical conductivity, electricity easily flowsin the solid solution and lithium is easily ionized. As a result, whenthe solid solution is used as the positive electrode active material,the amount of ions capable of entering and leaving the positiveelectrode active material is increased, and the capacity of the batterycan be increased.

In addition, according to an embodiment of the present invention,lithium metal oxide, which could not be used as a positive electrodematerial because lithium is contained therein but is less likely to beionized, can be used as a positive electrode material. Accordingly, alithium ion battery can be manufactured using lithium metal oxide, whichis excellent in terms of safety and cost but could not be used.

[Example 1]

In this example, a positive electrode active material and a positiveelectrode including the positive electrode active material, which areembodiments of the present invention, will be described with referenceto FIG. 1 and FIG. 2.

A positive electrode active material described in this example is formedusing a solid solution including lithium metal oxide with low electricalconductivity and lithium metal oxide with high electrical conductivity.

In this example, the solid solution was formed using Li₂MnO₃ includinglithium with a valence of 2 as the lithium metal oxide with lowelectrical conductivity and LiCo_(1/3)Mn₁nNi_(1/3)O₂ (LiCoMnNiO₄) as thelithium metal oxide with high electrical conductivity.

To form the solid solution, first, a mixture of positive electrodematerials is formed. In this example, Li₂Co₃, Co₃O₄, MnO₂, and NiO wereused as positive electrode materials. These positive electrode materialsare subjected to bail-mill treatment, whereby a mixture of positiveelectrode materials can be obtained.

Note that the ball-mill treatment was performed in this example underthe following conditions: acetone was used as an organic solvent; therotation rate was 400 rpm; the rotation time was two hours; and thediameter of a ball was φ3 mm. However, an embodiment is not limited tothis method as long as solid-phase reaction of the positive electrodematerials is promoted and the mixture of positive electrode materialswhich are uniformly fined can be obtained.

In addition, in order to promote reaction of the mixture of positiveelectrode materials by improving contact between the materials, pressureis applied to the mixture of positive electrode materials so that themixture of positive electrode materials is shaped into a pellet, andthen firing is performed thereon. The firing may be performed at atemperature higher than or equal to 600° C. and lower than or equal to1100° C., preferably higher than or equal to 900° C. and lower than orequal to 1000° C., for greater than or equal to one hour and less thanor equal to eight hours, preferably approximately five hours. In thisexample, the pellet was formed with a pressure of 14.7 MPa and thefiring was performed in an air atmosphere at 950° C. for five hours.

Through the steps, Li₂MnO₃—LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ which is a solidsolution having a layered structure can be obtained.

The above-described solid solution is mixed with a conductive agent, abinder, or the like and processed into a paste and the paste is appliedonto a collector and dried, so that a positive electrode precursor isformed. Note that titanium, aluminum, or the like can be used for thecollector. Pressure is applied to the positive electrode precursor andthe positive electrode precursor is shaped as needed, whereby a positiveelectrode is manufactured.

Note that as the conductive agent, an electron-conductive material whichdoes not cause chemical change in the power storage device may be used.For example, a carbon material such as graphite or carbon fibers, ametal material such as copper, nickel, aluminum, or silver, or a powderor a fiber of a mixture thereof can be used.

Note that as the binder, a polysaccharide, a thermoplastic resin, apolymer with rubber elasticity, and the like can be given. For example,starch, carboxymethyl cellulose, hydroxypropylcellulose, regeneratedcellulose, diacetylcellulose, poly vinyl chloride, polyvinylpyrrolidone,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM,styrene-butadiene rubber, butadiene rubber, fluorine rubber, or the likecan be used. In addition, polyvinyl alcohol, polyethylene oxide, or thelike may be used.

Alternatively, the positive electrode can be formed using a sputteringmethod. In the case of using a sputtering method, the positive electrodecan be formed in such a manner that a sputtering target is formed bysintering the positive electrode precursor and the sputtering target isintroduced into a sputtering apparatus.

In this case, a rare gas such as an argon gas may be used in thesputtering; alternatively, a nitrogen gas may be used. Furtheralternatively, a rare gas such as an argon gas and a nitrogen gas may beused in combination.

According to the above steps, the positive electrode active material andthe positive electrode including the positive electrode active materialcan be manufactured.

FIG. 1 is a graph of electrical conductivity of each of the lithiummetal oxide Li₂MnO₃, the lithium metal oxideLiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, and the solid solutionLi₂MnO₃—LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂. Note that the electricalconductivity was measured at room temperature while applying a pressureof 9.8 MPa after positive electrode precursors of the three materialswere formed. The electrical conductivity of Li₂MnO₃ was less than orequal to 10⁻¹⁰ S/cm, which was less than or equal to the measurementlimit; the electrical conductivity of LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ was1.45×10⁻⁶ S/cm; and the electrical conductivity of the solid solutionwas 1.58×10⁻⁷ S/cm. It can be found that the electrical conductivity ofthe solid solution is affected by LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂.

FIG. 2 is a graph showing a relation between voltage and capacity wheneach of the above-described three materials is used for a positiveelectrode of a lithium ion battery. FIG. 2 shows the values of thecapacities in the case of using Li₂MnO₃, LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂,and the solid solution Li₂MnO₃—LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, from theleft side.

The electrical conductivity of the lithium metal oxide Li₂MnO₃ isextremely low, and even when the lithium metal oxide Li₂MnO₃ is used asa positive electrode of a lithium ion battery, current does not flow andlithium cannot be utilized as a lithium ion, and therefore, the capacityis small. In addition, when the lithium metal oxideLiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, which has high electrical conductivity, isused, the capacity is somewhat large; however, since the valence oflithium is 1, any further increase of the capacity cannot be expected.

The solid solution Li₂MnO₃—LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ has highelectrical conductivity, which is affected by properties of the lithiummetal oxide LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂. Therefore, current easilyflows in the solid solution and lithium can be utilized as a lithiumion. The capacity can be further increased in the case where the lithiummetal oxide Li₂MnO₃ including lithium with a valence of 2 is used for apositive electrode as compared to that in the case of using only thelithium metal oxide LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ including lithium witha valence of 1 for a positive electrode because the lithium content inLi₂MnO₃ is larger than that in LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂ when bothhave the same number of moles.

With the use of the positive electrode active material and the positiveelectrode including the positive electrode active material which aredescribed in this example, a power storage device with large capacitycan be obtained.

[Example 2]

In this example, a battery which is an embodiment of the presentinvention and a manufacturing method thereof will be described. As apositive electrode of the battery, the positive electrode described inEmbodiment 1 and Example 1 is used.

FIG. 3 is a perspective view illustrating an example of a cylindricalstorage battery according to one embodiment of the present invention.Note that an embodiment of the present invention is not limited theretoand may be a square power storage device.

The cylindrical storage battery in FIG. 3 has a closed space surroundedby a battery sidewall 104, a battery cover 102, and a battery bottom106.

FIG. 4 is a cross-sectional view taken along a cross section 100 of thecylindrical storage battery in FIG. 3.

The battery sidewall 104 and the battery bottom 106 may be formed usinga conductive material and an appropriate material may be selected sothat the battery sidewall 104 and the battery bottom 106 haveappropriate mechanical strength and chemical resistance under the usageenvironment. For example, an aluminum alloy can be used. The closedspace is provided inside the battery surrounded by the battery sidewall104, the battery bottom 106, and the battery cover 102. An electrodebody 110 is placed in the closed space, for example.

The electrode body 110 is sandwiched between an insulating plate 112 onan upper portion (the battery cover 102 side) and an insulating plate114 on a lower portion (the battery bottom 106 side). A conductivewiring 120 and a conductive wiring 128 are drawn out from the insulatingplate 112 and the insulating plate 114, respectively. The conductivewiring 120 drawn out from the insulating plate 112 of the upper portion(the battery cover 102 side) is preferably connected to the batterycover 102 through a resistor 116. As the resistor 116, a heat sensitiveresistor whose resistance increases as a temperature rises is preferablyused. This is for prevention of abnormal heat generation due toexcessive current flow. The conductive wiring 128 drawn out from theinsulating plate 114 of the lower portion (the battery bottom 106 side)is connected to the battery bottom 106. Note that the battery bottom 106and the battery sidewall 104 are electrically connected to each other.

The battery sidewall 104, the battery cover 102, and the insulatingplate 112 of the upper portion (the battery cover 102 side) arepreferably connected to each other through a gasket 118. The gasket 118preferably has an insulating property; however, there is no limitationthereto and at least the battery cover 102 and the battery sidewall 104should be insulated from each other.

Although not illustrated, a structure may be employed in which a safetyvalve is provided inside the battery so that the connection between thebattery cover 102 and the electrode body 110 is cut off in the casewhere a negative electrode 126 and a positive electrode 122 areshort-circuited or the battery is heated and the pressure in the batteryincreases.

Further, a center pin may be inserted in the center of the electrodebody 110 in order to fix a position of the electrode body 110.

The electrode body 110 includes the negative electrode 126, the positiveelectrode 122, and a separator 124 provided therebetween. The positiveelectrode 122 of the electrode body 110 is electrically connected to thebattery cover 102 through the conductive wiring 120. The negativeelectrode 126 of the electrode body 110 is electrically connected to thebattery bottom 106 through the conductive wiring 128.

The negative electrode 126 is preferably formed using a collector and anactive material. For example, graphite or silicon serving as a negativeelectrode active material may be formed over a negative electrodecollector.

A negative electrode active material layer may be formed by mixing thenegative electrode active material with a conductive agent, a binder, orthe like and processed into a paste which is then applied onto acollector. Alternatively, the negative electrode active material layermay be formed by a sputtering method. Pressing may be also performed onthe negative electrode active material layer as needed.

Note that titanium, copper, or the like can be used for the collector.

Note that as the separator 124, paper, nonwoven fabric, a glass fiber, asynthetic fiber such as nylon (polyamide), vinylon (also called vinalon)(a polyvinyl alcohol based fiber), polyester, acrylic, polyolefin, orpolyurethane, or the like can be used. Note that a material which doesnot dissolve in an electrolyte solution should be selected.

As the electrolyte solution in which the separator 124 is soaked, forexample, a mixture in which lithium hexafluorophosphate (LiPF₆) is addedto a mixed solution of ethylene carbonate (EC) and diethyl carbonate(DEC) may be used. Further, as the electrolyte, lithium chloride (LiCl),lithium fluoride (LiF), lithium perchlorate (LiClO₄), lithiumfluoroborate (LiBF₄), lithium bis(trifluoromethanesulfonyl)imide(LiN(SO₂CF₃)₂), lithium bis(pentafluoroethanesulfonyl)imide(LiN(SO₂C₂F₅)₂), lithium trifluoromethansulfonate (LiCF₃SO₃), or thelike can be used. Furthermore, in the case where an alkali metal ionother than a lithium ion is used, sodium chloride (NaCl), sodiumfluoride (NaF), sodium perchlorate (NaClO₄), sodium fluoroborate(NaBF₄), potassium chloride (KCl), potassium fluoride (KF), potassiumperchlorate (KClO₄), potassium fluoroborate (KBF₄), or the like can beused, one or more of which may be dissolved in a solvent.

Note that examples of the solvent includes: cyclic carbonates such aspropylene carbonate (PC), butylene carbonate (BC), and vinylenecarbonate (VC); acyclic carbonates such as dimethyl carbonate (DMC),ethylmethyl carbonate (hereinafter abbreviated as EMC), methylpropylcarbonate (MPC), methylisobutyl carbonate (MIBC), and dipropyl carbonate(DPC); aliphatic carboxylic acid esters such as methyl formate, methylacetate, methyl propionate, and ethyl propionate; γ-lactones such asγ-butyrolactone; acyclic ethers such as 1,2-dimethoxyethane (DME),1,2-diethoxyethane (DEE), and ethoxymethoxy ethane (EME); cyclic etherssuch as tetrahydrofuran and 2-methyltetrahydrofuran; dimethylsulfoxide;1,3-dioxolane; alkyl phosphate esters such as trimethyl phosphate,triethyl phosphate, and trioctyl phosphate and fluorides thereof. Thesematerials can be used either alone or in combination.

Note that the case where a lithium ion is mainly included in theelectrolyte solution is described in this example; however, there is nolimitation thereto and another alkali metal ion may be used.

As described above, a battery can be manufactured using the electrodedescribed in Embodiment 1 as the positive electrode.

With the structure described in this example, a power storage devicehaving large capacity can be obtained.

This application is based on Japanese Patent Application serial no.2010-064427 filed with Japan Patent Office on Mar. 19, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A power storage device comprising: apositive electrode comprising a solid solution, wherein the solidsolution comprises a first alkali metal oxide and a second alkali metaloxide, wherein the solid solution has an electrical conductivity greaterthan or equal to 1 ×10 ⁻⁷ S/cm and less than or equal to 10 ×10 ⁻⁷ S/cm,wherein the first alkali metal oxide is Li₂MnO₃, wherein the secondalkali metal oxide is LiCo_(1/3) Mn_(1/3) Ni_(1/3)O₂, and wherein thepositive electrode is formed by a sputtering method by using a targetcomprising the solid solution.
 2. The power storage device according toclaim 1, wherein the solid solution has a layered structure, and whereinthe solid solution is Li₂MnO₃—LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂.
 3. A powerstorage device comprising: a positive electrode comprising a solidsolution, wherein the solid solution comprises a first alkali metaloxide and a second alkali metal oxide, wherein the solid solution has anelectrical conductivity greater than or equal to 1 ×10 ⁻⁷ S/cm and lessthan or equal to 10 ×10⁻⁷ S/cm, wherein the first alkali metal oxide isany of Li₂MnSiO₄ and Li₂FeSiO₄, wherein the second alkali metal oxide isany of LiCoO₂, LiMn₂O₄, and LiNiO₂, and wherein the positive electrodeis formed by a sputtering method by using a target comprising the solidsolution.
 4. A power storage device comprising: a first conductivewiring electrically connected to a battery cover; a second conductivewiring electrically connected to a battery bottom which is insulatedfrom the battery cover; and an electrode body provided with an upperinsulating plate and a lower insulating plate, wherein the electrodebody comprises: a positive electrode comprising a solid solution, andelectrically connected to the first conductive wiring through the upperinsulating plate; a negative electrode electrically connected to thesecond conductive wiring through the lower insulating plate; and aseparator provided between the positive electrode and the negativeelectrode, wherein the solid solution comprises a first alkali metaloxide and a second alkali metal oxide, wherein the solid solution has anelectrical conductivity greater than or equal to 1 ×10⁻⁷ S/cm and lessthan or equal to 10 −10 ⁻⁷ S/cm, wherein the first alkali metal oxide isany of Li₂MnSiO₄ and Li₂FeSiO₄, wherein the second alkali metal oxide isany of LiCoO₂, LiMn₂O₄, and LiNiO₂, and wherein the positive electrodeis formed by a sputtering method by using a target comprising the solidsolution.
 5. The power storage device according to claim 1, wherein thepositive electrode further comprises a conductive agent and a binder. 6.The power storage device according to claim 5, wherein the conductiveagent is a graphite of a carbon fiber.
 7. The power storage deviceaccording to claim 3, wherein the positive electrode further comprises aconductive agent and a binder.
 8. The power storage device according toclaim 7, wherein the conductive agent is a graphite of a carbon fiber.9. The power storage device according to claim 4, wherein the positiveelectrode further comprises a conductive agent and a binder.
 10. Thepower storage device according to claim 9, wherein the conductive agentis a graphite of a carbon fiber.
 11. A sputtering target comprising asolid solution and a conductive agent, wherein the solid solution isLi₂MnO₃—LiCo_(1/3)Mn_(1/3) Ni_(1/3)O₂, and wherein the solid solutionhas an electrical conductivity greater than or equal to 1 ×10⁻⁷ S/cm andless than or equal to 10 ×10⁻⁷ S/cm.
 12. The sputtering target accordingto claim 11, wherein the conductive agent is a graphite of a carbonfiber.